1. Field of the Invention
The present invention relates to the fabrication of semiconductor integrated circuit structures and, in particular, to the use of an oxygen implant technique for generating completely isolated silicon islands.
2. Discussion of the Related Art
Fabrication of semiconductor integrated circuits involves the electrical interconnection of a large number of individually insulated circuit elements through specific interconnect paths. Thus, integrated circuit fabrication necessarily involves steps for electrically isolating the individual silicon "active areas" into which the circuit elements are built.
Perhaps the best known of the techniques that have been developed for isolating device elements is local oxidation of silicon (LOCOS) isolation. LOCOS isolation involves the formation of semi-recessed oxide in the non-active (or "field") regions of the silicon substrate.
Circuits that are of a higher circuit element density, or that must function under high voltages or in harsh environments, require more stringent isolation techniques. These techniques are generally categorized as silicon-on-insulator (SOI) isolation methods.
FIGS. 1A-1C illustrate an example of one currently-utilized SOI technique. First, as shown in FIG. 1A, oxygen ions 12 are implanted to a desired depth in the silicon substrate 10 at about 2 MeV. The silicon substrate 10 is then heat treated to cause the implanted oxygen to react with surrounding silicon to form an in-grown silicon oxide layer 14 in the bulk silicon, as shown in FIG. 1B. Trenches are then anisotropically etched into the substrate 10 to the depth of the in-grown oxide layer 14. The trenches are then filled with deposited oxide 16 which is planarized (e.g., by chemical mechanical polishing) to expose the surface of the silicon 10, as shown in FIG. 1C, thereby providing isolated silicon active device regions 18.
U.S. Pat. No. 5,346,841, issued Sep. 13, 1994, to Kotaro Yajima, discloses an isolation technique that also uses oxygen ion implantation. In the Yajima process, a mask for blocking ion implantation is formed on a silicon substrate. Oxygen ions are then implanted at an angle of about 45.degree. while the substrate is rotated in a plane parallel to the substrate surface. This results in a conically shaped implant of oxygen ions into the silicon substrate. Subsequent heat treatment of the substrate results in oxidation of the silicon in the implanted areas to form oxide isolation for conically shaped silicon active device regions.